Schottky barrier diode and method for making the same

ABSTRACT

A schottky diode includes a SiC substrate which has a first surface and a second surface facing away from the first surface, a semiconductor layer which is formed on the first surface of the SiC substrate, a schottky electrode which is in contact with the semiconductor layer, and an ohmic electrode which is in contact with the second surface of the SiC substrate. The first surface of the SiC substrate is a (000-1) C surface, upon which the semiconductor layer is formed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 13/150,635 filedJun. 1, 2011, which is a Division of U.S. Ser. No. 11/975,366, filedOct. 18, 2007, which application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a schottky barrier diode and a methodof making the same.

2. Description of Related Art

FIG. 6 is a conventional schottky barrier diode (hereinafter called“schottky diode.”) (See JPA-2004-221513 for example.) The schottky diodeX shown in this figure uses a SiC substrate 91 as a basis. The SiCsubstrate 91 has a (0001) Si surface 91 b and a (000-1) C surface 91 a.The (0001) Si surface 91 b is formed with an epitaxial layer 92. Theepitaxial layer 92 is formed with a guard ring 92 a. On the epitaxiallayer 92, an insulation layer 93, a schottky electrode 94 and a metallayer 95 are formed in lamination. A contact hole 93 a is made in theinsulation layer 93 to provide contact between the schottky electrode 94and the epitaxial layer 92. The contact made by the schottky electrode94 and the epitaxial layer 92 is schottky contact. The (000-1) C surface91 a is formed with an ohmic electrode 96. The contact made by the ohmicelectrode 96 and the SiC substrate 91 is ohmic contact. The schottkydiode X is used to provide a relatively high-speed switching in electriccircuits.

In the schottky diode X, the schottky barrier height between theschottky electrode 94 and the epitaxial layer 92 must have anappropriate value in order to reduce leak current caused by reversevoltage. However, in the above-described design where the epitaxiallayer 92 is formed on the (0001) Si surface 91 b, the best achievableschottky barrier height was about 1.0 eV, and for this reason, it hasnot been possible in the schottky diode X to sufficiently reduce theleak current caused by reverse voltage.

SUMMARY OF THE INVENTION

The present invention has been proposed under the above-describedcircumstance, and it is therefore an object of the present invention toprovide a schottky diode which has a reduced leak current caused byreverse voltage, and to provide a method for making the schottky diode.

A schottky diode provided by a first aspect of the present inventionincludes: a SiC substrate which has a first surface and a second surfacefacing away from the first surface; a semiconductor layer which isformed on the first surface of the SiC substrate; a schottky electrodewhich is in contact with the semiconductor layer; and an ohmic electrodewhich is in contact with the second surface of the SiC substrate. Thefirst surface of the SiC substrate is a (000-1) C surface, and thesemiconductor layer is formed on this (000-1) C surface.

According to such an arrangement as the above, the schottky barrierheight between the schottky electrode and the epitaxial layer has avalue large enough to reduce the leak current, and therefore it ispossible to decrease the leak current in the schottky diode.

Preferably, the schottky electrode is made of Mo. Such an arrangement asthe above makes it possible to achieve appropriate ohmic contact betweenthe schottky electrode and the epitaxial layer.

Preferably, the barrier height between the semiconductor layer and theschottky electrode is 1.3 through 1.7 eV. Such an arrangement as theabove makes it possible to reduce the leak current to about 1/10 of theleak current as compared to a case where the epitaxial layer is formedon the (0001) Si surface of the SiC substrate.

A second aspect of the present invention provides a method for making aschottky diode. This method includes a step of forming a semiconductorlayer on a (000-1) C surface of a SiC substrate; a step of forming anohmic electrode on a (0001) Si surface of the SiC substrate; and a stepof forming a schottky electrode on the semiconductor layer after thestep of forming an ohmic electrode.

Preferably, the method according to the present invention furtherincludes a step of performing a heat treatment at a temperature between200 through 1,000° C. to a laminated body provided by the SiC substrate,the semiconductor layer, the ohmic electrode and the schottky electrodeafter forming the schottky electrode.

Such an arrangement as the above is suitable for achieving the schottkybarrier height which has a value between 1.3 through 1.7 eV.

Other characteristics and advantages of the present invention willbecome clearer from the following detailed description to be made withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a primary portion of a schottky diodeaccording to the present invention.

FIG. 2 is a sectional view showing a step of forming an epitaxial layeras part of a method for making the schottky diode.

FIG. 3 is a sectional view showing a step of forming an insulation layeras part of the method.

FIG. 4 is a sectional view showing a step of forming an ohmic electrodeas part of the method.

FIG. 5 is a sectional view showing a step of forming a schottkyelectrode as part of the method.

FIG. 6 is a sectional view showing a primary portion of a conventionalschottky diode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described belowwith reference to the drawings.

FIG. 1 shows a schottky diode according to the present invention. Theillustrated schottky diode A includes a SiC substrate 1, an epitaxiallayer 2, an insulation layer 3, a schottky electrode 4, a metal layer 5,and an ohmic electrode 6. The schottky diode A is a type of rectifierimplemented by using a schottky barrier formed by a junction between ametal and a semiconductor, and has advantages over rectifiersimplemented by using a PN junction, in that voltage drop in normaldirection is smaller and switching speed is higher.

The SiC substrate 1 is an n⁺ type 4H—SiC substrate doped with impurityto an extent that the substrate has a resistivity of 0.01 through 0.05Ωm. The SiC substrate 1 has a (000-1) C surface 1 a and a (0001) Sisurface 1 b.

The epitaxial layer 2 is made of an n⁻ type Si C of a relatively highlevel of purity, with its impurity density of 1.0×10¹⁵ through 2.0×10¹⁶cm⁻³ for example.

The epitaxial layer 2 is formed on the (000-1) C surface 1 a of thesubstrate 1. The epitaxial layer 2 is formed with a guard ring 21. Theguard ring 21 is an annular portion formed by ion injection using suchan element as Al and B into a part of the epitaxial layer 2, andenhances voltage endurance of the schottky diode A.

The insulation layer 3 is a member formed of e.g. SiO₂ or Si₃N₄. Theinsulation layer 3 provides partial coverage on each of the epitaxiallayer 2 and the guard ring 21. The insulation layer 3 has a contact hole3 a. The contact hole 3 a exposes the inner circumferential edge of theguard ring 21.

The schottky electrode 4 is made of e.g. Mo, and works to form aschottky barrier between itself and the epitaxial layer 2. The schottkyelectrode 4 is in contact with part of the epitaxial layer 2 and part ofthe guard ring 21 exposed by the contact hole 3 a of the insulationlayer 3. In the present embodiment, the schottky barrier height betweenthe schottky electrode 4 and the epitaxial layer 2 is 1.3 through 1.7eV.

The metal layer 5 is made of Al for example, and covers the schottkyelectrode 4. A layer of e.g. Ti or MoN may be formed between the metallayer 5 and the schottky electrode 4.

The ohmic electrode 6 is formed on the (0001) Si surface 1 b of the SiCsubstrate 1, and works as an electrode to inject electrons into theschottky diode A. The ohmic electrode 6 is used to solder the schottkydiode A to e.g. a circuit substrate. In order to enable appropriatesoldering, the ohmic electrode 6 is formed to have a multilayerstructure provided by e.g. Ti, Ni and Ag.

Next, a method for making the schottky diode A will be described withreference to FIG. 2 through FIG. 5.

First, as shown in FIG. 2, a SiC substrate 1 is prepared. The SiCsubstrate 1 is made by e.g. sublimation method or HTCVD method. Next, anepitaxial layer 2 is formed on the (000-1) C surface 1 a of the SiCsubstrate 1. The formation of the epitaxial layer can be performed byepitaxially growing an n⁻ type SiC by supplying row material gas(propane gas, silane gas), carrier gas (H₂ gas), and dopant gas (N₂ gas)toward the (000-1) C surface 1 a, for example.

Next, as shown in FIG. 3, a guard ring 21 is formed. The guard ring 21can be formed through ion injection of e.g. Al, B, etc. into part of theepitaxial layer 2 to a depth of 0.1 through 1.0 μm. After forming theguard ring 21, an insulation layer 3 is formed. The insulation layer 3can be formed by e.g. spattering with SiO₂ or Si₃N₄.

Next, an ohmic electrode 6 is formed on the (0001) Si surface 1 b of thesubstrate 1. The ohmic electrode is formed by making layers of Ti, Niand of Ag through spattering or vapor deposition. Through this step, alaminated body (1, 2, 21, 3, 6) shown in FIG. 4 is obtained. After theformation of the ohmic electrode 6, a heat treatment is performed inwhich the laminated body is exposed to an ambient temperature not lowerthan 900° C. for a time not shorter than a minute. Through this heattreatment, state of contact between the ohmic electrode 6 and the SiCsubstrate 1 becomes ohmic.

After the heat treatment which has yielded the ohmic contact, a contacthole 3 a is made as shown in FIG. 5, in the insulation layer 3 by meansof etching for example. Then, spattering or vapor depositing isperformed to form layers of Mo and of Al. The Mo layer and the Al layerare then patterned lithographically to form a schottky electrode 4 and ametal layer 5. Through this step, a second laminated body (1, 2, 21, 3,4, 5, 6) shown in FIG. 5 is obtained.

Thereafter, the second laminated body undergoes a heat treatment inwhich the laminated body is exposed to an ambient temperature of 200° C.through 1,000° C. According to experiments conducted by the inventor etal., a schottky barrier height of about 1.5 eV was achievable in a heattreatment at an ambient temperature of 450° C. for 20 minutes. Also, aschottky barrier height of about 1.4 eV was achievable in a heattreatment at an ambient temperature of 450° C. for 100 minutes.

Through the steps described above, a schottky diode A shown in FIG. 1 isobtained. It should be noted here that a preferred temperature range forthe heat treatment performed after the formation of the schottkyelectrode 4 is 200 through 1,000° C. This ambient temperature range issuitable for adjusting a schottky barrier height to 1.3 through 1.7 eV.

Next, description will be made for functions of the schottky diode A andthe method for making it.

As described, the schottky diode A has a schottky barrier height of 1.3through 1.7 eV. With the schottky barrier height being such, it ispossible to reduce the leak current in the schottky diode A than before.For example, the leak current in the schottky diode A can be about 1/10of the leak current in a conventional schottky diode which has aschottky barrier height of about 1.0 eV.

Unlike the present invention, the construction shown in FIG. 6 where theepitaxial layer 92 is formed on the (0001) Si surface 91 b can onlyachieve the schottky barrier height of up to 1.0 through 1.1 eV. On thecontrary, according to the present embodiment, the epitaxial layer 2 isformed on the (000-1) C surface 1 a of the substrate 1. This makes itpossible to achieve a relatively large value of about 1.3 through 1.7 eVfor the schottky barrier height between the epitaxial layer 2 and theschottky electrode 4.

The schottky electrode 4 formed of Mo is suitable for creating schottkycontact between the electrode and the epitaxial layer 2.

According to the method for making the schottky diode A, a heattreatment for creating ohmic contact is performed at a relatively hightemperature (900° C. or higher), and the schottky electrode 4 is formedthereafter. This eliminates a risk that the schottky barrier height willbe unduly lowered by the heat treatment.

The invention claimed is:
 1. A method for making a schottky diode, themethod comprising: preparing an SiC substrate including a first surfaceand a second surface opposite to the first surface; forming asemiconductor layer on the first surface of the SiC substrate; forming aguard ring in the semiconductor layer by ion injection; forming aninsulation layer on the semiconductor layer, the insulation layercovering the guard ring; forming an ohmic electrode on the secondsurface of the SiC substrate, thereby providing a first laminated bodymade up of the SiC substrate, the semiconductor layer, the guard ring,the insulation layer and the ohmic electrode; subjecting the firstlaminated body to a first heat treatment; forming an opening in theinsulation layer after the first heat treatment, thereby exposing a partof the semiconductor layer and a part of the guard ring through theopening; forming a schottky electrode and a metal layer covering theschottky electrode, thereby providing a second laminated body made up ofthe first laminated body, the schottky electrode and the metal layer,the schottky electrode being configured to cover the exposed pan of thesemiconductor layer and the exposed part of the guard ring; andsubjecting the second laminated body to a second heat treatment.
 2. Themethod according to claim 1, wherein the first surface of the SiCsubstrate comprises a (000-1) C surface.
 3. The method according toclaim 1, wherein the second surface of the SiC substrate comprises a(0001) Si surface.
 4. The method according to claim 1, wherein theforming of the semiconductor layer is performed by epitaxially growingan n⁻ type SiC.
 5. The method according to claim 1, wherein the formingof the guard ring is performed by ion injection of Al or B into aportion of the semiconductor layer.
 6. The method according to claim 1,wherein the first heat treatment comprises exposing the first laminatedbody to an ambient temperature of not lower than 900° C. for at leastone minute.
 7. The method according to claim 1, wherein the schottkyelectrode is made of Mo, and the metal layer is made of Al.
 8. Themethod according to claim 1, wherein the second heat treatment comprisesexposing the second laminated body to an ambient temperature in a rangeof 200 to 1,000° C.
 9. The method according to claim 1, wherein thesecond heat treatment comprises exposing the second laminated body to anambient temperature of 450° C. for 20 minutes.
 10. The method accordingto claim 1, wherein the second heat treatment comprises exposing thesecond laminated body to an ambient temperature of 450° C. for 100minutes.